Posted By paulw71
Hi David
Personnally speaking I do not think there is a Definitive size, suffice to say that should it be neccessary for someone to enter it for maintenance, cleaning or other purposes along with the possibility that someone may need to be rescued from it. the design must take this into account.
As always I could be wrong

Posted By Dave Daniel
Not sure about the circumstances of the access requirement. I came across something like this some years ago regarding the inspection and cleaning of road tankers (milk lorries actually). We came to the conclusion and showed that the inspection requirement could be met using CCTV inspection systems of the pipeline type without entering. The cleaning requirement (build-up of "milk stone" - Limescale) was a lesser requirement which was contracted out and then reduced/eliminated by more agressive automated cleaning which was part of the regular cleaning process.

Any use?